This invention relates broadly to nuclear fuel elements for use in the core of nuclear fission reactors. More particularly, the present invention relates to a method for making a zirconium containing composite cladding for nuclear fuel having a metal coating on its inner surface in proximity to the fuel and an intermediate zirconium oxide boundary layer.
Nuclear reactors are presently being designed, constructed and operated in which the nuclear fuel is contained in fuel elements which can have various geometric shapes, such as plates, tubes, or rods. The fuel material is usually enclosed in a low neutron absorbing corrosion-resistant, non-reactive, heat conductive container or cladding. The fuel elements are assembled together in a lattice at fixed distances from each other in a coolant flow channel or region forming a fuel assembly, and sufficient fuel assemblies are combined to form the nuclear fission chain reacting assembly or reactor core capable of a self-sustained fission reaction, The core in turn is enclosed within a reactor vessel through which a coolant is passed. The cladding serves several purposes and two primary purposes are: first, to prevent contact and chemical reactions between the nuclear fuel and the coolant or the moderator if a moderator is present, or both if both the coolant and the moderator are present; and second, to prevent the radioactive fission products, some of which are gases, from being released from the fuel into the coolant or the moderator or both if both the coolant and the moderator are present. Common cladding materials are steel and its alloys, zirconium and its alloys, niobium (columbium) and its alloys, and the like. The failure of the cladding, i.e., a loss of the leak tightness, can contaminate the coolant or moderator and the associated systems with radioactive fission products to a degree which interferes with plant operation.
Problems have been encountered in the manufacture and in the operation of nuclear fuel elements which employ certain metals and alloys as the clad material due to mechanical or chemical reactions of these cladding materials under certain circumstances. Zirconium and its alloys, under normal circumstances, are excellent nuclear fuel claddings since they have low neutron absorption cross sections, are strong, ductile, extremely stable, and at temperatures below about 750.degree. F. (about 398.degree. C.), are non-reactive in the presence of demineralized water and/or steam which are commonly used as reactor coolants and moderators.
However, fuel element performance has revealed a problem with mechanical interactions between the nuclear fuel and the cladding in the presence of certain fission products produced by nuclear fission reactions. It has been discovered that this undesirable performance is promoted by localization of mechanical stresses (due to fuel-cladding differential expansion) at cracks and at pellet-to-pellet interfaces in the nuclear fuel. Corrosive fission products are released from the nuclear fuel and are present at pellet-to-pellet interfaces and at the intersection of fuel cracks with the cladding surface. Fission products are created in the nuclear fuel during the fission chain reaction during operation of the nuclear reactor. The localized stress is exaggerated by high friction between the fuel and the cladding.
An approach to fuel design has been to coat the nuclear fuel material with a ceramic to prevent moisture coming in contact with the nuclear fuel material as disclosed in U.S. Pat. No. 3,108,936. U.S. Pat. No. 3,085,059 discloses a fuel element including a metal casing containing one or more pellets of fissionable ceramic material and a layer of vitreous material bonded to the ceramic pellets. Thus, the layer is provided between the casing and the nuclear fuel to assure uniformly good heat conduction from the pellets to the casing. U.S. Pat. No. 2,873,238 describes jacketed fissionable slugs of uranium canned in a metal case and having zinc-aluminum bonding layers as protective jackets or coverings between the slugs and the case. U.S. Pat. No. 2,849,387 discloses jacketed body sections of nuclear fuel which have been dipped into a molten bath of a bonding material giving an effective thermally conductive bond between the uranium body sections and the container (or cladding). The coating is disclosed as any metal alloy having good thermal conduction properties with examples including aluminum-silicon and zinc-aluminum alloys. Japanese Patent Publication No. SHO 47-14200 discloses a fuel element in which one of two groups of pellets is coated with a layer of silicon carbide and the other group is coated with a layer of pyrocarbon or metal carbide.
The coating of a nuclear fuel material introduces reliability problems in that achieving uniform coatings free of faults is difficult. Further, the deterioration of the coating involves loss of performance of the nuclear fuel element.
U.S. patent application Ser. No. 330,152, filed Feb. 6, 1973 now abandoned, discloses a method for preventing failure of nuclear fuel cladding consisting of the addition of a metal such as niobium to the fuel. The additive can be in the form of a powder, provided the subsequent fuel processing operation does not oxidize the metal, or the additive can be incorporated into the fuel element as wires, sheets or other forms in, around, or between fuel pellets.
Document GEAP-4555, dated February, 1964, describes a composite cladding of a zirconium alloy with an inner lining of stainless steel metallurgically bonded to the zirconium alloy. The composite cladding is fabricated by extrusion of a hollow zirconium alloy billet having an inner lining of stainless steel. This cladding has the disadvantage that the stainless steel layer involves a neutron absorption penalty of about ten to fifteen times the penalty for a zirconium alloy layer of the same thickness.
U.S. Pat. No. 3,502,549 discloses a method of protecting zirconium and its alloys by the electrolytic deposition of chromium thereon to provide a composite material useful for nuclear reactors. A method for electrolytic deposition of copper on Zircaloy-2 surfaces and subsequent heat treatment for the purpose of obtaining diffusion of the copper into the plated Zircaloy-2 surface is disclosed in Energia Nucleare, Volume 11, number 9 (September 1964) at pages 505-508.
Stability and Compatibility of Hydrogen Barriers Applied to Zirconium Alloys, by F. Brossa et al (European Atomic Energy Community, Joint Nuclear Research Center, EUR 4098e 1969), describes methods of deposition of different coatings on zirconium alloys and the efficiency of these coatings as hydrogen diffusion barriers. An Al-Si coating is stated to be the most promising barrier against hydrogen diffusion.
Methods for electroplating nickel on zirconium and zirconium tin alloys and heat treating these alloys to produce alloy-diffusion bonds are disclosed in Electroplating on Zirconium and Zirconium-Tin, by W. C. Schickner et al (BM1-757, Technical Information Service, 1952). U.S. Pat. No. 3,625,821 discloses a fuel element in which the cladding tube is coated on its inner surface with a burnable poison retaining metal of low neutron capture cross section such as nickel and having finely dispersed particles of a burnable poison retained therein.
Reactor Development Program Progress Report of August, 1973 (ANL-RDP-19) discloses a sacrificial layer of chromium as a chemical getter arranged on the inner surface of a stainless steel cladding.
Another approach to provide cladding protection has been to introduce a barrier between the nuclear fuel material and the cladding, as disclosed in U.S. Pat. No. 3,230,150 (copper foil), German Patent Publication DAS No. 1,238,115 (titanium layer), U.S. Pat. No. 3,212,988 (sheath of zirconium, aluminum or beryllium), U.S. Pat. No. 3,018,238 (barrier of crystalline carbon between the UO.sub.2 and the zirconium cladding, and U.S. Pat. No. 3,088,893 (stainless steel foil). While the barrier concept proves promising, some of the foregoing references involve materials which are incompatible with the nuclear fuel (e.g., carbon can combine with oxygen from the nuclear fuel).
Other fuel clad barrier concepts are disclosed in U.S. Pat. No. 3,969,186 (refractory metal such as molybdenum, tungsten, rhenium, niobium and alloys thereof in the form of a tube or foil of single or multiple layers or a coating on the internal surface of the cladding), and U.S. Pat. No. 3,925,151 (liner of zirconium, niobium or alloys thereof between the nuclear fuel and the cladding with a coating of a high lubricity material between the liner and the cladding).
Another fuel clad barrier for protecting the zirconium or zirconium alloy cladding container is shown in the Gordon, et al U.S. Pat. No. 4,029,545 assigned to the same assignee as the present invention. In that patent, a layer such as chromium is electroplated onto a zirconium or zirconium alloy substrate, followed by the electroplating of a metal layer selected from copper, nickel or iron onto the chromium layer. However, it has been found to be uneconomical to electroplate the zirconium or zirconium alloy cladding with chromium thus rendering this patent less promising than originally anticipated. An alternative procedure is shown by Gordon et al in U.S. Pat. No. 4,022,662 in which a zircaloy material encloses a free standing metal tube comprised of either stainless steel, copper or copper alloys or nickel or nickel alloys that in turn encloses the core of nuclear fuel material. A diffusion barrier comprised of a chromium coating is coated on either the inside surface of the cladding or on the outside surface of the metal tube. Again, the Gordon et al nuclear fuel element is uneconomical because chromium electrodeposition is required and a separate copper tube has to be fabricated. Therefore it is still desirable to achieve an economic solution of the problem of preventing perforations or failures in nuclear fuel cladding resulting from metal embrittlement or stress corrosion cracking involving fuel pelletcladding interaction.
One very successful approach is disclosed in U.S. patent application Ser. No. 820,797, filed Aug. 1, 1977 in the names of William T. Grubb and Lawrence H. King entitled "Nuclear Fuel Element Having a Composite Coating" and assigned to the same assignee as the present invention, and now abandoned in favor of continuation-in-part application Ser. No. 087,547, filed Oct. 22, 1979. The nuclear fuel element consists of a core of nuclear fuel material enclosed in zirconium or zirconium alloy container, the inside surface of which is coated first with an intermediate zirconium oxide diffusion barrier layer and then with a metal selected from the group consisting of copper, nickel or iron. This patent application of Grubb, et al also discloses a method of making such a container comprising the steps of:
(A) etching or roughening the inner surface of the zirconium or zirconium alloy container,
(B) oxidizing the etched or roughened surface to produce a zirconium oxide coating,
(C) activating the zirconium oxide coating by contacting the coating with salts of tin or various noble and precious metals to permit the metallic coating of such surface by electroless deposition, and
(D) further coating the activated zirconium oxide layer with a metal.